CDS (I) 2022 General Knowledge · Question 74 of 120

Correct — B, Flow of the South-East Trade Winds towards the Indian Ocean. That is the ordinary state of the equatorial Pacific, not something El Nino brings about. NOAA's own description of normal conditions is that the trade winds blow west along the equator, carrying warm surface water from South America towards Asia, while cold water rises to replace it off the South American coast. During El Nino the opposite happens to the winds: they weaken, and the warm water that they had been piling up in the west is pushed back east toward the Americas. So a strong westward flow of the trades toward the Indian Ocean side of the Pacific describes the non-El Nino condition; if anything, El Nino slackens it. Each of the other three options is a genuine consequence — the atmospheric circulation is distorted, the pattern of evaporation shifts with the warm water, and the collapse of coastal upwelling starves the plankton on which the fishery depends.

• (a)Distortion of equatorial atmospheric circulation — This is a resultant. El Nino displaces the rising limb of the Walker circulation eastward across the Pacific, which is precisely a distortion of the equatorial atmospheric circulation, and it is the mechanism by which the event reaches Indian and Australian rainfall.
• (c)Irregularities in the evaporation of sea water — Also a resultant. Evaporation follows the warm surface water; when the warm pool shifts east, the belt of intense evaporation and deep convection shifts with it, and rainfall follows the moisture.
• (d)Reduction in the amount of planktons which reduces the number of fish in the sea — A resultant too, and the one after which the phenomenon is named. Peruvian fishermen noticed the warm water because it stopped the upwelling that brings nutrients to the surface; without nutrients the plankton fail and the anchovy fishery collapses.

El Nino is the warm phase of the El Nino-Southern Oscillation, a coupled ocean-atmosphere cycle of the tropical Pacific. NOAA defines it as a periodic warming of sea-surface temperatures across the central and east-central equatorial Pacific, and declares an episode when the three-month average sea-surface temperature departure in the Nino 3.4 region exceeds 0.5 degrees Celsius. Episodes typically last nine to twelve months and recur every two to seven years. The name comes from the Spanish for the boy child; Peruvian fishermen from the 1600s called it El Nino de Navidad because the warming peaks around December.

The question is really about direction of causation. Three options describe things that change when El Nino arrives; one describes the machinery that was already running before it. The reliable test is to ask, for each option, whether it is true in a normal year — if it is, it cannot be a resultant. The trades blowing westward is the normal state, so option (b) fails that test at once. It is worth being careful with the wording as well: the option says the trades flow towards the Indian Ocean, which is the westward direction across the Pacific toward the maritime continent, and that is what El Nino weakens rather than creates. For India the practical consequence is that El Nino years tend to bring a weaker south-west monsoon, while La Nina years tend to strengthen it. As of the April 2022 exam the Pacific was in a La Nina phase; NOAA declared El Nino conditions again on 8 June 2023, and that event ran through the 2023 Indian monsoon.

• El Nino is a phenomenon of the Pacific Ocean, not of the Indian Ocean.
• In normal conditions the trade winds blow westward along the equator, moving warm water toward Asia and allowing cold, nutrient-rich water to upwell off South America.
• During El Nino the trade winds weaken and warm water is pushed back east toward the Americas, shutting down that upwelling.
• NOAA declares an El Nino when the three-month mean sea-surface temperature anomaly in the Nino 3.4 box exceeds 0.5 degrees Celsius; events last about nine to twelve months and recur every two to seven years.
• El Nino is generally associated with a weaker south-west monsoon over India, and La Nina with a stronger one.

• Placing El Nino in the Indian Ocean; it is a Pacific phenomenon, and the Indian Ocean has its own dipole.
• Treating the westward trade winds as an El Nino effect; they are the normal condition that El Nino disrupts.
• Assuming every El Nino year gives India a drought; the correlation is real but far from perfect, and the Indian Ocean Dipole can cut across it.

As a which-is-not-a-consequence item like this one, as a comparison with La Nina, or as a statements question on where El Nino forms and how it is measured.

• practice — not a real PYQ

  During an El Nino event, the trade winds over the equatorial Pacific

  • (a)strengthen and blow more steadily westward
  • (b)weaken and may even reverse
  • (c)shift to a north-south direction
  • (d)are unaffected

  Answer(b) weaken and may even reverse — which is why warm water moves back east and coastal upwelling off South America shuts down.

• practice — not a real PYQ

  El Nino is best described as a phenomenon of which one of the following oceans?

  • (a)The Indian Ocean
  • (b)The Atlantic Ocean
  • (c)The Pacific Ocean
  • (d)The Southern Ocean

  Answer(c) The Pacific Ocean — NOAA defines it as a periodic warming of sea-surface temperatures across the central and east-central equatorial Pacific.